The present invention relates to drill assemblies, and particularly to backheads and check valves used in drill assemblies having fluid-actuated pistons.
Drill assemblies, particularly percussive down-hole drills having fluid-actuated pistons, are well known, such as those disclosed in U.S. Pat. No. 5,085,284 of Fu, U.S. Pat. No. 5,301,761 of Fu et al., U.S. Pat. No. 5,562,170 of Wolfer et al., U.S. Pat. No. 5,711,205 (Wolfer et al.) and U.S. Pat. No. 5,566,771 of Wolfer et al. Referring to FIGS. 1 and 2, a typical down-hole drill assembly 1 includes a casing 2 containing the internal components of the drill assembly 1, including a piston 3 and a backhead 4 that connects the drill assembly 1 to a drilling machine (not shown). The backhead 4 has a supply passage 5 for directing percussive fluid, generally high-pressure compressed air, into the drill assembly 1. The upper end 4a of the backhead 4 is connected with a source of percussive fluid, for example, a fluid supply line connected with a compressor (not shown), such that percussive fluid is supplied to the drill assembly 1 to operate the piston 3.
The piston 3 is slidably mounted within the casing 2 and is guided by an inner bearing surface 2a of the casing 2 so as to reciprocally impact with a drill bit 6, which provides the work output of the drill assembly 1. The piston 3 is actuated by the pressure of the percussive fluid, which is directed alternately to the upper and lower ends 3a, 3b, respectively, of the piston 3 so as to thereby cause reciprocating sliding movement of the piston 3. Typically, the drill assembly includes a fluid distributor 7 having at least one passage 8, preferably a plurality of passages 8, for directing percussive fluid to a drive chamber (not shown) defined within the casing 2 between the distributor 7 and the upper end 3a of the piston 3. The flow of percussive fluid through the distributor passages 8 is preferably regulated by a distributor valve 10 disposed about and slidable along a cylindrical end portion 7a of the distributor 7. However, such a distributor valve 10, while enabling ideal operation of the drill assembly 1, is not required for the drill assembly 1 to function in an appropriate manner.
In order to prevent contaminants, such a water or rock debris, from entering or "backflowing" into the casing 2 and potentially causing damage to and/or interfering with the proper functioning of the components of the drill assembly 1, the pressure of the percussive fluid in the casing 2 should always remain above ambient pressure. However, when the supply of percussive fluid to the backhead 4 is turned off, percussive fluid within the casing 2 will flow out of the drill assembly 1 through the backhead supply passage 5, if the passage 5 is not sealed, allowing the backflow of contaminants into the drill assembly 1.
One known device to prevent backflow into the drill assembly 1 is to provide a check valve 11 disposed within the casing 2 between the fluid distributor 7 and the backhead 4, as best shown in FIG. 2. The check valve 11 includes a valve plug 12 that is movable toward and away from a valve seating surface 4b, which in FIGS. 1 and 2 is provided by a tapered wall portion inside the backhead 4 surrounding an axial opening 4c that forms a section of the supply passage 5. The plug 12 is biased toward the seating surface 4a by a valve spring 13 extending between the movable plug 12 and a stationary valve cap 14. The valve cap 14 includes a first recess 14a at an upper side which retains the lower end of the spring 13 and a second recess 14b on the lower side, which is sized to receive the end of the distributor portion 7a. Further, to maintain the distributor 7 at a desired position within the casing 2, a retainer member 16 is biased by a spring 17 to push against the valve cap 14, such that the distributor 7 is "sandwiched" between the valve cap 14 and the upper end of a sleeve 18 (FIG. 1).
Due to the fact that the above-described check valve 11 has several moving parts, at least some components of the valve 11 will eventually have to be replaced. The valve spring 13 will likely experience fatigue failure after a certain period of use. Further, the movable plug 12 may become worn due to repeated impact with the valve seating surface 4a, such that percussive fluid may flow or "seep" between the plug 12 and the seat 4b and cause the casing pressure to decrease to ambient pressure. Thus, the valve spring 13 and movable plug 12 may have to be replaced several times during the life of the drill assembly 1. Further, due to the potential that one or more check valve components will wear or break, there is still a significant risk that the backflowing of contaminants may occur.
In view of the above-discussed limitations with the known check valve 11, it would be desirable to have a check valve system that involved fewer moving parts. Further, it would be desirable to have a check valve that was simpler and thus less expensive to produce. Furthermore, it would also be desirable to have a device for maintaining the distributor 7 at a desired position that is simpler than the spring-biased retainer 16 and that preferably has no moving parts.